In an optical information reader for reading out information recorded on a digital audio disk, a video disk, or the like, rotational driving of a recording disk is controlled by a spindle servo device, and recorded information is read out by a pickup movably provided on the disk in the radial direction thereof. Further, there are provided a focus servo device in which control is performed for correctly converging an information detecting spotlight from the pickup on a recording surface of the disk, and a tracking servo device in which control is peformed for causing the spotlight to track a recording track, for example, spirally formed on the recording surface of the disk. In the tracking servo device, the higher the loop gain, the higher the trackability (a capability of following a track) of the information detecting spotlight so that a malfunction such as so-called track jumping is not caused even if there are external vibrations, or if the disk has substantially eccentricity (i.e. is not perfectly round.) As a result, stable tracking control becomes possible.
However, in information readers having these kinds of servo devices, it is possible that an abnormal portion such as a defect, a scar, or the like, on the recording surface of the disk can cause distortion of an error signal of the servo system, resulting in a malfunction such as track jumping or the like when the information reading spotlight from the pickup tracks the recording track at the abnormal portion.
To solve this problem, it has been proposed that in a tracking servo device, when an abnormal portion is detected on the recording surface, a servo operation be performed with a reduced loop gain or with an immediately preceding error value, or alternatively that a servo operation is to be performed instead on the basis of a tracking error signal independent of the abnormal point obtained by another way, thereby obtaining a stable tracking operation without causing any malfunction such as track jumping due to a defect, such as a scar, on the recording surface.
As to reading of the recorded information at the abnormal portion such as a defect or a scar, on the recording surface, on the other hand, when drop-outs due to a defect, a scar, or the like are detected, a drop-out compensation circuit (not shown) prevents the recorded information from being produced so that it is unnecessary to perform particular control onto a focus servo device for an abnormal portion on the recording surface.
However, experiments by the inventors have shown that disturbance in a focus servo system due to a defect such as a scar as described above in turn disturbs a tracking servo system. That is, when a defect is detected on a recording surface of a disk, a sufficiently stable tracking operation cannot be performed unless the tracking servo system is controlled in addition to controlling the focus servo system.